2. Field of the Invention
This invention relates to an image reproducing apparatus.
2. Related Background Art
As recent standards of television broadcasting, in addition to a conventional NTSC system having an aspect ratio of 3:4 and 525 scan lines, there has been proposed a more precise television system such as an HDTV system having an aspect ratio of 9:16 and 1125 scan lines which system is intended to be used in practice. This HDTV system has image information about five times as much as the NTSC system, and the screen is wide. Thus, fine and outstanding images can be obtained.
The amount of information is great so that it is impossible to transmit the information as it is to each house, because of restrictions of broadcasting waves and communication capacity. Therefore, image information is transmitted while using a so-called distribution quality lower than the original image quality at a studio (i.e., having a narrower bandwidth), and also compressing the bandwidth. Table 1 shows examples of various factors of studio quality and distribution quality of the HDTV system.
Moving images of the HDTV at each house are essentially limited to the distribution quality.
Apart from the above, since the HDTV system uses a larger number of scan lines and has fine images with less flicker, not only moving images but also still images are fine and outstanding, leaving possible applications to arts, culture, education, animation and other fields. In general, television viewers look at a single still image for several seconds to ten and several seconds so that the quality of a still image is desired to have the studio quality.
In this regard, in addition to an HDTV moving image transmission/reception (reproduction) apparatus, an HDTV still image transmission/reception (reproduction) apparatus has been proposed. FIG. 7 is a block diagram showing the structure of a still image receiving apparatus. A receiver 10 receives a broadcasting wave from an antenna and derives therefrom a bandwidth-compressed still image information signal. A still image decoder 12 decodes the compressed still image signal outputted from the receiver 10 and expands its bandwidth to thereby output original still image data. The image data outputted from the decoder 12 is developed and stored as a still image in a frame memory 14 under control of a frame memory controller 16. After the necessary image data is stored in the frame memory 14, the frame memory controller 16 reads the stored data at a faster scan rate. The read data is converted into an analog signal by a D/A converter 18, and applied to a monitor 20 which then displays the received image.
An output from a power source 22 is connected via a power source switch 24 to the receiver 10, decoder 12, frame memory 14, frame memory controller 16, D/A converter 18, and monitor 20. When the power source switch 24 is turned off, the circuits 10 to 20 becomes inactive.
TABLE 1 ______________________________________ Studio Distribution Quality Quality ______________________________________ Scan Lines 1125 lines 1125 lines Brightness (Y) 30 MHz 20 MHz Brightness Sampling 74.25 MHz 48.6 MHz Frequency Color Difference 15 MHz 7 MHz Bandwidth (Pr, Pb) Color Difference 33.125 MHz 16.2 MHz Sampling Frequency ______________________________________
With the conventional apparatus shown in FIG. 7, if still images are transmitted at a relatively long interval (e.g., several ten seconds), a still image may not be displayed on a monitor for a long time after the power source switch is turned on.
If power is always supplied at least to the circuits 10, 12 and 16, a broadcast image can be displayed on the monitor 20 immediately after the power source switch is turned on. Such arrangement is however not practical because of a large power consumption by the decoder 12 and frame memory.
If the power source switch is to be manually turned on or off, it must be turned on before a still image data is transmitted and turned off after the end of broadcasting, requiring cumbersome user activity. In addition, turning on or off the power source switch may be inadvertently missed. If the apparatus is maintained by being turned on for a long period without noticing it, the apparatus may become overheated and dangerous.